Photographic images are traditionally printed on photographic paper using conventional film based optical printers. The photographic industry, however, is converting to digital imaging. The process uses images obtained from digital cameras, or scanned film exposed in traditional photographic cameras, to create digital image files that are then printed onto photographic paper.
The growth of the digital printing industry has led to multiple approaches to digital printing. One of the early methods used for digital printing was cathode ray tube (CRT) based printers such as the Centronics CRT recorder. Another commonly used approach to digital printing is the laser based engine shown in U.S. Pat. No. 4,728,965. Such systems are generally polygon flying spot systems which use red, green, and blue lasers.
A more contemporary approach uses a single spatial light modulator such as a Texas Instruments digital micromirror device (DMD) as shown in U.S. Pat. No. 5,061,049. Spatial light modulators provide significant advantages in cost as well as allowing longer exposure times, and have been proposed for a variety of different printing systems from line printing systems such as the printer depicted in U.S. Pat. No. 5,521,748, to area printing systems such as the system described in U.S. Pat. No. 5,652,661.
Another low cost solution uses LCD modulators. Several photographic printers using commonly available LCD technology are described in U.S. Pat. Nos. 5,652,661, 5,701,185, and 5,745,156. Most of these designs involve the use of a transmissive LCD modulator such as is depicted in U.S. Pat. Nos. 5,652,661 and 5,701,185.
A problem with all of these devices is that they occasionally produced defects in the final image. These defects may be due to, for example, surface finishing of the LCD which results in a high spatial frequency defects. Another defect, again using the LCD as an example, is low spatial frequency defects in the resulting image caused by warping of the plate as it is attached to the LCD during manufacture.
It is the purpose of this invention to correct image defects in an area spatial light modulator based printing system.